MySQL Cookbook, 2nd Edition

Sequencing an Unsequenced Table

Problem

You forgot to include a sequence column when you created a table. Is it too late to sequence the table rows?

Solution

No, just add an AUTO_INCREMENT column using ALTER TABLE. MySQL will create the column and number the rows automatically.

Discussion

To add a sequence to a table that doesn’t currently contain one, use ALTER TABLE to create an AUTO_INCREMENT column. Suppose that you have a table t that contains name and age columns, but no sequence column:

+----------+------+
| name     | age  |
+----------+------+
| boris    |   47 |
| clarence |   62 |
| abner    |   53 |
+----------+------+

You can add a sequence column named id to the table as follows:

mysql>ALTER TABLE t
    -> ADD id INT NOT NULL AUTO_INCREMENT,
    -> ADD PRIMARY KEY (id);
mysql> SELECT * FROM t ORDER BY id;
+----------+------+----+
| name     | age  | id |
+----------+------+----+
| boris    |   47 |  1 |
| clarence |   62 |  2 |
| abner    |   53 |  3 |
+----------+------+----+

MySQL numbers the rows for you automatically. It’s not necessary to assign the values yourself. Very handy.

By default, ALTER TABLE adds new columns to the end of the table. To place a column at a specific position, use FIRST or AFTER at the end of the ADD clause. The following ALTER TABLE statements are similar to the one just shown, but place the id column first in the table or after the name column, respectively:

ALTER TABLE t
  ADD id INT NOT NULL AUTO_INCREMENT FIRST,
  ADD PRIMARY KEY (id);

ALTER TABLE t ADD id INT NOT NULL AUTO_INCREMENT AFTER ...